$A$ metal nitrate reacts with $KI$ to give a black precipitate which on addition of excess of $KI$ converts into an orange-colored solution. The cation of the metal nitrate is

• A
  $Hg^{2+}$
• B
  $Bi^{3+}$
• C
  $Pb^{2+}$
• D
  $Cu^{+}$